Fermented plant juice (or FPJ) is one of the live cultures, or inputs, used in Korean natural farming to provide both microbes and nutrients to plants. If you have ever made sauerkraut by lacto-fermentation, the process is similar.
The following is only a basic description, not a specific method. Please research further and expect to do some trial and error if you choose to make FPJ.
Gather fast growing green shoots from nutrient rich plants one hour before daylight (timing is for maximum nutrient content).
Cut the plant material into short pieces (an inch or two / a few centimeters). Don't wash (dew that may be present is okay)
Weigh the plant material
Place plant material into a bucket or mixing bowl
Weigh the plant material and add an equal amount of brown sugar (i.e. one kilogram of plant material and one kilogram of brown sugar)1
Knead the mixture of plant and sugar with hands until uniformly mixed and somewhat crushed
Pack the mixture into fermentation vessel (crock or bucket), pressing down to remove air. A weight may be useful to hold contents down, as gases develop (I use a small plate with a stone).
Cover opening with cheesecloth or tea towel, (even paper will work) and secure with elastic or string. This will keep fruit flies and gnats out. Don't seal, the fermentation releases gases.
Cover the whole container with a large towel or store in a dark place. Light is not beneficial to fermentation.
Fermentation time varies with temperature. If ambient temperature is high (you are sweating), it may take as little as five days. If ambient temp is low (you have sweater on), it may take two weeks.
If you are not sure whether your FPJ is ready, taste it. As fermentation progresses, the sweetness of the mix will decrease and the acidity will increase. Once it has a sweet and sour character, squeeze as much liquid as you can from the material. I like to use the same cloth with which I covered the jar for this. The left over plant solids, once liquid has been removed, can be added to compost or buried in the garden. Place the liquid in a suitable container (again, don't seal).
FPJ may be either used to enrich soil when watering plants or sprayed directly onto their leaves. Dilution rate is from 800 parts clean water to 1 part FPJ to more than 1000 to 1. A little of this stuff goes a long way!. It will store indefinitely but gets even stronger with age. The masters say to mix a little fresh FPJ with the old and dilute even more. Spray diluted mixture on leaves to point of runoff. Best time to do this is just before sundown.
For those wanting to try this, Chris (no relation) Trump has a series of YouTube videos on Korean natural farming. Here is the step by step demo on making Fermented plant juice (FPJ).
1. The proportion of brown sugar to plant matter varies in different tutorials. I have used 2 parts plant to 1 part brown sugar and had good results.